The invention relates generally to optical transmission systems using wavelength division multiplexing, WDM, and particularly to an optical amplification unit used therein.
In optical transmission systems, an optical signal is modulated with an outbound data stream, and the modulated optical signal is applied to optical fiber. In order to increase the capacity of the system, the bandwidth of the data stream can be increased or more wavelengths can be introduced, each of which is modulated with a discrete data stream. The latter method is termed wavelength division multiplexing.
Wavelength division multiplexing (WDM) is an efficient way of multiplying the capacity of optical fiber. In wavelength division multiplexing, several independent transmitter-receiver pairs use the same fiber. FIGS. 1a and 1b illustrate the principle of wavelength division multiplexing, using as an example a system having four parallel transmitter-receiver pairs. Each of the four information sources (not shown in the figure) modulates one of four optical transmitters, each of which generates light at a different wavelength (xcex1 . . . xcex4). As will be seen from FIG. 1a, the modulation bandwidth of each source is smaller than the distance between the wavelengths, and thus the spectra of the modulated signals do not overlap. The signals generated by the transmitters are combined onto the same optical fiber OF in a WDM multiplexer WDM1, which is a fully optical (and often passive) component. At the opposite end of the fiber, a WDM demultiplexer WDM2, which is also a fully optical (and often passive) component, separates the different spectral components of the combined signal from one another. Each of these signals is detected at a discrete receiver. Hence, a narrow wavelength window is assigned for the use of each signal in a given wavelength range. A typical practical example might be a system where the signals are in the 1550 nm wavelength range for example in such a way that the first signal is at wavelength 1544 nm, the second signal at wavelength 1548 nm, the third signal at wavelength 1552 nm and the fourth signal at wavelength 1556 nm. Nowadays a multiple of 100 GHz (approx. 0.8 nm) is becoming the de facto standard for the distance between wavelengths.
Erbium-doped fiber amplifier (EDFA) has generally been used as an optical amplifier on optical fiber links, since it combines several good properties, such as an overall simple structure and the availability of reliable and effective pump lasers. In these amplifiers, the amplification takes place in Er-doped fiber (for which the term active fiber will be used hereinafter), but also other kinds of doping have been used when for example amplification in another wavelength range has been desired.
However, such amplifiers are not very suitable as such for implementing WDM links, since their uneven amplification curve places restrictions on the selection of wavelengths. For this reason, amplifiers with a flattened amplification curve are nowadays generally used on WDM links. In other words, the amplification curve must be flattened so that the different wavelengths experience substantially equal amplification. To flatten the amplification curve, either (1) a filter evening out the amplification differences can be incorporated into the EDFA, or (2) the active fiber in the amplifier can be exchanged for fiber having a flatter amplification curve. Such fiber is for example erbium-doped fluoride fiber, for which reason such amplifiers are called erbium-doped fluoride fiber amplifiers, EDFFA.
In such an amplifier that is common to all wavelengths, the entire output power must be divided among all wavelengths, wherefore in practice an upper limit exists for the amplification experienced by each signal, said limit being the lower the more of these signals of different wavelengths the WDM signal contains. Furthermore, a significant drawback in using a filter is that the filter possesses a specific spectral form that has been designed with the assumption that the unevenness of the amplification has a specific format as a function of the wavelength. If the power input to the amplifier deviates from its hypothetical value, also the form of the gain curve (which is dependent on the power) will change, and thus the operation of the filter can become very unfavourable. In view of power consumption, it is also very disadvantageous that the wavelengths experiencing the greatest amplification must be attenuated with the filter.
On the other hand, the practical implementation of EDFFA is very difficult on account of the fact that the amplifier uses fiber material differing considerably from the material used in conventional telecommunications fibers. For this reason, joining the fibers to each other, for example, is very difficult. On account of these difficulties, practical implementations of EDFFA hardly exist. EDFFA has also poorer noise characteristics than EDFA, and EDFFA amplifiers cannot be pumped at different wavelengths like EDFA amplifiers.
An alternative to a single common amplifier is to use a dedicated amplifier for each wavelength of the WDM signal, by means of which the entire output of each amplifier is acquired for the use of the signal concerned. In such an implementation, the WDM signal must first be demultiplexed in order to separate the different wavelengths for amplification. The solution is quite expensive, as it requiresxe2x80x94in addition to a demultiplexer and parallel amplifiersxe2x80x94multiplexer means by which the pump signal needed for amplification and each wavelength channel signal in the WDM signal are multiplied onto the fiber of the amplifier corresponding to said wavelength channel signal. (In this context, the term pump signal is used, even though mere optical pump power carrying no data is concerned.)
It is an object of the invention to eliminate the drawbacks described above and to provide an amplifier solution that is advantageous in view of power utilization and furthermore can be implemented more simply and cost-effectively than heretofore.
This object is achieved with a solution as defined in the independent claims.
The idea of the invention is to use a waveguide phased array component at the output or input end or both ends of the amplifier unit for processing both wavelength channel signals and pump signals. For the input end, for example, this means that the same component is used as a demultiplexer separating the different wavelengths of the WDM signal and also as a multiplexer combining the pump signal with each wavelength channel signal of the WDM signal. The waveguide phased array component (whereof also the term waveguide array grating or arrayed waveguide grating is used) is a known component used in fiber optics and is highly suitable for systems using wavelength division multiplexing for example for the reason that a large number of different wavelengths can be transported therethrough.
By means of the invention, the basic solution of several parallel amplifiers, in which the entire output power of one amplifier is acquired totally for the use of a single wavelength channel signal (wavelength), is considerably simplified, since the processing of the wavelength channel signals contained in the WDM signal and the pump signals can be effectively integrated, so that the internal redundancy of the amplifier unit will be diminished.
On account of the solution in accordance with the invention, a very high amplification can be achieved for each wavelength, or alternatively, if a smaller amplification is sufficient, a very simple amplifier can be used for each signal, which will result in a more cost-effective amplifier unit.
One significant additional advantage of the solution in accordance with the invention is that the amplifier unit can be implemented as a very compact structure, as will be described hereinafter. At best, the solution enables integration of the entire amplifier unit into the same component onto the same substrate).
A further advantage of the invention is that each wavelength channel signal can be easily measured (unlike in a conventional amplifier), which allows wavelength-specific monitoring to be carried out.